Let’s explore how population size can change over time and how various factors can influence the size. Before you get started, don’t forget to print out your OnTRACK Biology Journal.

TEKS Standards and Student Expectations

B(12) The student knows that biological systems work to achieve and maintain balance. The student is expected to:

B(12)(D) describe how events and processes that occur during ecological succession can change populations and species diversity

Learning Objectives

Describe different factors that can affect population growth.

Define carrying capacity.

Explain how carrying capacity is related to population size.

Distinguish between exponential population growth and logistic population growth.

Describe how humans can impact populations and species diversity.

Essential Questions

What are different factors that affect population growth?

What is carrying capacity and how does it relate to population growth?

What conditions are necessary for a population to exhibit exponential growth?

How can humans impact populations and species diversity?

Vocabulary

• Population
• Immigration
• Emigration
• Exponential Growth
• Carrying Capacity
• Logistic Growth

A population is a group of individuals of the same species that share the same area. You already know that organisms are born and die within a given population every year, but how does studying the birth rate and death rate help us understand the long-term survival of a species? Species survive over thousands of years by acquiring resources that are necessary for survival. In nature, these resources are often limited, and individuals within a population must compete for resources in order to survive.

Long-term survival of a species is based on obtaining resources that are limited, so that individuals can live and reproduce. What are some resources that organisms must compete for in nature? Click on each plus sign below to find out.

Directions: Watch Populations: Biotic Potential to learn more about how populations are affected by resources and their environment.

In Populations: Biotic Potential, you learned about four factors affecting population growth:

• Births
• Deaths
• Immigration
• Emigration

In many natural populations, immigration and emigration contribute relatively little to population size, so birth and death rates are the primary influence on population size. When conditions are ideal and organisms have unlimited resources, populations can exhibit exponential growth. When a population grows exponentially, there are many more births than deaths, and the population increases rapidly over time.

Directions: Click on the Growth of E. coli Bacteria Population graph and the Growth of Snowy Owl Population graph to see what exponential growth looks like over time.

With exponential growth, the number of organisms added in each generation increases as the population gets larger. The number of offspring produced is always greater than the generation before. You may notice that the population stays small for a period of time, and then it rapidly increases to a large number of individuals.

Populations may experience periods of exponential growth in nature, but they always reach a final tipping point where limited resources can no longer support the it. At this point, the population has reached its carrying capacity. For example, when a population of zebras exceeds the amount of water available, zebras will begin to die. These deaths will bring the population numbers below the carrying capacity. As the water is replenished, zebras will survive and live to reproduce, allowing the population to grow. This cycle is repeated over and over again throughout the long-term survival of the species.

Directions: Click on the graph below to see a zebra population that hovers around the population's carrying capacity.

The red line, which represents the population size in hundreds of zebras over time, will continue to weave above and below carrying capacity. This shows that the population is in dynamic equilibrium. If individuals are added to the population above and beyond the carrying capacity, the population size will decrease. If the population is smaller than the carrying capacity, it will increase. The population growth shown in this graph is also called logistic growth. The population initially shows a period of exponential growth, slows as resources are limited by population size, and eventually stabilizes in dynamic equilibrium.

Human beings, either directly or indirectly, can have a big impact on the resources needed by different species to survive. Click on the pictures below for some examples.

Directions: Watch Biological Carrying Capacity for a look at how human beings can manage natural resources to support the overall health of certain species, and the ecosystem where they live. After viewing, respond to questions 1–9 in the OnTRACK Biology Journal.